Process for Producing (Meth)Acrylic Acid or (Meth)Acrolein

ABSTRACT

An object of the present invention is to provide a method for producing (meth)acrylic acid or (meth)acrolein by conducting a gas phase catalytic oxidation reaction with an oxygen-containing gas using as a raw material at least one substance to be oxidized selected from propylene, propane, isobutylene and (meth)acrolein using a multi-tubular reactor, which enables a high yield and stable production even when operating constantly with supplying the raw material in the maximum supply amount acceptable by the reactor or an amount close thereto. 
     The invention is a method for producing (meth)acrylic acid or (meth)acrolein wherein, at the time of a start-up of the reaction, for a period of at least 20 hours or more after the supply amount of the raw material to the reactor per unit time reached 30% or more of the acceptable maximum supply amount of the raw material per unit time, the supply amount of the raw material per unit time is kept at 30% or more and less than 80% of the acceptable maximum supply amount.

TECHNICAL FIELD

The present invention relates to a method for producing (meth)acrylicacid or (meth)acrolein by a gas phase catalytic oxidation of at leastone substance to be oxidized selected from propylene, propane,isobutylene and (meth)acrolein with a molecular oxygen using amulti-tubular reactor stably and efficiently.

BACKGROUND ART

(Meth)acrylic acid or (meth)acrolein is produced by a gas phasecatalytic oxidation reaction in which propylene, propane, isobutylene or(meth)acrolein is brought into contact with a molecular oxygen or amolecular oxygen-containing gas in the presence of a composite oxidecatalyst. This gas phase catalytic oxidation reaction is conductedusually with a multi-tubular reactor.

In such a reaction system, it is a matter of course that it is desirableto obtain an intended material stably at a high yield.

Based on the findings obtained newly by inventors of this invention, the(meth)acrylic acid or (meth)acrolein can be obtained stably at a highyield by using a certain contrivance at the time of a start-up of thereaction described above.

As a contrivance at the time of a start-up in a reaction systememploying a catalytic gas phase oxidation reactor, a start-up methodwhich is safe and can recycle its exhausted gas is proposed in PatentReference 1 (JP-A-2001-53519). Patent Reference 2 (JP-A-2003-265948)also proposes a method for effecting a start-up efficiently without anyadverse effect on the activity of a catalyst in a shell-tube typereactor which circulates a heat medium which is solid at ambienttemperature.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a method for producing(meth)acrylic acid or (meth)acrolein by conducting a gas phase catalyticoxidation reaction with an oxygen-containing gas using as a raw materialat least one substance to be oxidized selected from propylene, propane,isobutylene and (meth)acrolein using a multi-tubular reactor, whichenables a high yield and stable production even when operatingconstantly with supplying the raw material in the maximum supply amountacceptable by the reactor or an amount close thereto.

The inventors of this invention discovered that (meth)acrylic acid or(meth)acrolein can be produced more stably at a higher yield byemploying a procedure in which, at the time of a start-up of thereaction, for a period of at least 20 hours or more after the supplyamount of the raw material per unit time (hereinafter sometimes simplyreferred to as “supply amount”) reached 30% or more of the acceptablemaximum supply amount of the raw material to the reactor undergoing astationary operation, the supply amount of the raw material is kept atthe amount less than 80% of the acceptable maximum supply amount, andhave achieved this invention based on this finding.

Thus, this invention is a method for producing (meth)acrylic acid or(meth)acrolein by conducting a gas phase catalytic oxidation reactionwith an oxygen-containing gas using as a raw material at least onesubstance to be oxidized selected from propylene, propane, isobutyleneand (meth)acrolein using a multi-tubular reactor, wherein, at the timeof a start-up of the reaction, for a period of at least 20 hours or moreafter the supply amount of the raw material to the reactor per unit timereached 30% or more of the acceptable maximum supply amount of the rawmaterial per unit time, the supply amount of the raw material per unittime is kept at 30% or more and less than 80% of the acceptable maximumsupply amount.

In a prior method for producing acrolein for example from propylene,about 20 hours is required, at the time of the start-up, for raising thepropylene supply amount from 30% to 100% of the acceptable maximumsupply amount to the reactor.

Based on the inventors' researches, a conventional start-up allows somereaction tube to exhibit an abnormally elevated temperature, followed bya reduction in temperature (a rapid increase in the peak temperaturefollowed by the loss of the temperature peak is observed by using athermocouple). Such findings mean that a part which exhibits aspecifically high activity (hereinafter referred to as an activityspecific point) is present in the layer of a catalyst packed in areaction tube and this activity specific point exhibits a highreactivity at the time of a start-up to cause a rapid increase in thetemperature by which a surrounding catalyst is affected to result in adeactivation of the catalyst in the entire reaction tube (a loss of thetemperature peak). Actually, when initiating the reaction by aconventional start-up and conducting a stationary operation with theacceptable maximum supply amount or a supply amount close thereto, thereaction yield was reduced by about 4% and the differential pressurebetween the inlet and the outlet of a deactivated reaction tube washigher by 3 times or more than that of a normal reaction tube. Such adeactivated reaction tube still remained at a frequency of about 5%after the stationary operation for one year.

On the contrary, when a time period during which the propylene supplyamount is kept at 30% or more and less than 80%, for example kept at 70%of the acceptable maximum supply amount after reaching 30% of theacceptable maximum supply amount was 20 hours or more, for example, 10days, according to a method of the invention, there was no reaction tubeexhibiting an abnormally elevated temperature as described above. Asubsequent stationary operation with the acceptable maximum supplyamount or a supply amount close thereto for one year resulted in thedifferential pressure between the inlet and the outlet of a reactiontube which was same to that at the time of the initiation of theoperation, with the catalytic activity being stable without undergoingany deactivation. The reaction yield was improved to about 2% whencompared with a prior art. This may be attributable to a sufficient timeperiod provided until a stationary operation near the acceptable maximumsupply amount, which allows an activity specific point to disappearwithout affecting a surrounding catalyst.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic sectional view indicating one embodiment of amulti-tubular heat exchange reactor employed in a gas phase catalyticoxidation method of the invention, FIG. 2 shows a schematic viewindicating one embodiment of a baffle employed in a multi-tubular heatexchange reactor according to the invention, FIG. 3 shows a schematicview of another embodiment of the baffle which is different from that inFIG. 2, FIG. 4 shows a schematic sectional view of another embodiment ofa multi-tubular heat exchange reactor employed in a gas phase catalyticoxidation method of the invention which is different from that in FIG.1, FIG. 5 shows a magnified schematic sectional view of an intermediatetube plate which divides a shell in the multi-tubular heat exchangereactor shown in FIG. 4.

In this connection, the reference numerals 1 b and 1 c in the drawingsare reaction tubes, 2 is a reactor, 3 a and 3 b are circular conduits, 3a′ and 3 b′ are circular conduits, 4 a is a product outlet, 4 b is a rawmaterial supply inlet, 5 a and 5 b are tube plates, 6 a and 6 b areholed baffles, 6 a′ and 6 b′ are holed baffles, 7 is a circulation pump,8 a and 8 a′ are heat medium supply lines, 8 b and 8 b′ are heat mediumdraining line, 9 is an intermediate tube plate, 10 is a heat shieldingplate, 11, 14 and 15 are thermometers, 12 is a stagnation space, and 13is a spacer rod.

BEST MODE FOR CARRYING OUT THE INVENTION

The invention is detailed below.

The method of the invention is a method for producing (meth)acrylic acidor (meth)acrolein by conducting a gas phase catalytic oxidation reactionusing a multi-tubular reactor packed with a catalyst effecting a gasphase catalytic oxidation using as a raw material at least one substanceto be oxidized selected from propylene, propane, isobutylene and(meth)acrolein, characterized in that a sufficient time period isprovided until a stationary operation near the acceptable maximum supplyamount by adjusting the raw material supply amount at the time of astart-up of the reaction.

As used herein, the term “acceptable maximum supply amount” means amaximum amount of a raw material allowed to be supplied to a reactor perunit time. This value correlates with the production capability of thereactor and determined at the stage of the designing of the reactor.

In the invention, at the time of a start-up of the reaction, for aperiod of at least 20 hours or more, preferably 24 hours or more and notmore than 80 hours, after the supply amount reached 30% or more of theacceptable maximum supply amount, the supply amount of the raw materialto the reactor is kept at 30% or more and less than 80%, preferably 50%or more and not more than 75%, as mentioned above. As a result, theeffects of the invention can be exerted and the (meth)acrylic acid orthe (meth)acrolein can be produced stably at an improved yield even whenoperating with the acceptable maximum supply amount.

The reaction systems, reactors, catalysts and the like employed in theinvention are described below.

(Reaction Systems)

While a representative of the reaction systems in an industrial methodfor producing an acrolein and acrylic acid may for example be a one-pathsystem, an unreacted propylene recycling system and a combustion exhaustgas recycling system, the invention is not limited to any reactionsystems including these three systems.

(1) One-Path System:

In this system, propylene, air and steam are supplied as being mixed ata former stage reaction to convert mainly into an acrolein and acrylicacid, the outlet gas of which is supplied to a latter stage reaction(mainly converting the acrolein to the acrylic acid) without separatingfrom the products. In this procedure, it is common to supply air andsteam required in the reaction at the latter stage in addition to theoutlet gas from the former stage, to the reaction at the latter stage.

(2) Unreacted Propylene Recycling System:

In this system, the reaction product gas containing the acrylic acidobtained in the latter stage reaction is introduced into acrylic acidtrapping device, where the acrylic acid is trapped as an aqueoussolution, and a part of the exhausted gas containing unreacted propylenein this acrylic acid trapping device is supplied to the former stagereaction, whereby recycling a part of the unreacted propylene.

(3) Combustion Exhaust Gas Recycling System:

In this system, the reaction product gas containing the acrylic acidobtained in the latter stage reaction is introduced into acrylic acidtrapping device, where the acrylic acid is trapped as an aqueoussolution, all of the exhausted gas in this acrylic acid trapping deviceis combusted and oxidized to convert the contained unreacted propyleneand the like mainly into carbon dioxide and water, and a part of thecombusted exhausted gas thus obtained is added to the former stagereaction.

Generally, a multi-tubular reactor is used for the purpose of increasingthe productivity of the reactor while protecting a catalyst and keepingthe performance of the catalyst at a high level by controlling thecatalyst reaction temperature precisely due to an extremely largeexothermic heat as in an oxidation reaction.

Recently, the amount of the production of acrylic acid from propyleneand methacrylic acid from isobutylene (together referred to as(meth)acrylic acid) was increased greatly in response to an increaseddemand, and a large number of plants were built in the world, with theplant production scale being increased to 100 thousand tons or more perplant per year. As a result of an increased plant production scale, theamount produced by a single oxidation reactor should be increased,resulting in an increased load of a gas phase catalytic oxidationreactor of propane, propylene or isobutylene. Accordingly, amulti-tubular reactor is needed to be imparted with a far higherperformance.

In the invention, a method for a gas phase catalytic oxidation of asubstance to be oxidized with a molecular oxygen-containing gas using amulti-tubular reactor having, in its longitudinal direction of thereaction tubes, a cylindrical reactor shell having a raw material supplyinlet and a product outlet, a plural of circular conduits provided onthe outer circumference of the cylindrical reactor shell for allowing aheat medium to come into or go out of the cylindrical reactor shell, acirculating device connecting the plurality of the circular conduitswith each other, a plural of reaction tubes which are restrained by aplural of the tube plate of the reactor and which contain catalysts anda plural of baffles for changing the direction of the heat mediumallowed to come into the reactor shell is employed, and the reactiontube described above is packed with an oxidation catalyst such as anMo—Bi-based catalyst and/or an Mo—V-based catalyst.

The invention is a gas phase catalytic oxidation method which employspropylene, propane, isobutylene or (meth)acrolein or a mixture thereofas a substance to be oxidized and a gas phase catalytic oxidation isconducted using a molecular oxygen-containing gas to obtain(meth)acrolein or (meth)acrylic acid. (Meth)acrolein, (meth)acrylic acidor both are obtained from propylene, propane, isobutylene. (Meth)acrylicacid is obtained also from (meth)acrolein.

As used herein, a “process gas” means a gas involved in a gas phasecatalytic oxidation reaction including a substance to be oxidized and amolecular oxygen-containing gas as raw material gases, resultantproducts and the like. A “raw material” means a substance to beoxidized.

(Raw Material Gas Composition)

To a multi-tubular reactor employed in a gas phase catalytic oxidation,a gas mixture of at least one substance to be oxidized selected frompropylene, propane, isobutylene and (meth)acrolein, a molecularoxygen-containing gas and water vapor is mainly introduced as a rawmaterial gas.

In the invention, the concentration of the substance to be oxidized inthe raw material gas is 6 to 10% by mole, with the oxygen being in anamount of 1.5 to 2.5 molar times and the water vapor in an amount of 0.8to 5 molar times that the substance to be oxidized. The raw material gasintroduced passes through the reaction tubes as being divided into eachreaction tube, and reacts in the presence of the packed oxidizingcatalyst.

(Multi-Tubular Reactor)

A gas phase catalytic oxidation reaction according to the inventionwhich employs a multi-tubular reactor is a method employed widely forproducing (meth)acrylic acid or (meth)acrolein using a molecular oxygenor a molecular oxygen-containing gas in the presence of a compositeoxide catalyst from at least one substance to be oxidized selected frompropylene, propane, isobutylene and (meth)acrolein.

A multi-tubular reactor employed in the invention is not limitedparticularly and may be one employed industrially.

One embodiment of a multi-tubular reactor employed in the invention isdescribed with referring to FIGS. 1 to 5.

(FIG. 1)

FIG. 1 shows a schematic sectional view indicating one embodiment of amulti-tubular heat exchange reactor employed in a gas phase catalyticoxidation method of the invention.

In a shell 2 of the multi-tubular reactor, reaction tubes 1 b and 1 care fixed on tube plates 5 a and 5 b. A raw material supply inlet whichis an inlet of raw material gas for the reaction and a product outletwhich is an outlet of a product are 4 a or 4 b. While the direction ofthe flow of a process gas may be in any way when the flows of a processgas and a heat medium are countercurrents, 4 b is the raw materialsupply inlet in FIG. 1 since the direction of the flow of the heatmedium in the reactor shell is indicated upward by an arrow. On theouter circumference of the reactor shell, circular conduits 3 a forallowing the heat medium to come in are provided. The heat medium whosepressure has been raised by a circulation pump 7 for the heat mediumgoes from the circular conduits 3 a up through the reactor shell withits direction of the flow being converted due to a plural ofalternatively located holed baffles 6 a having openings near the centerof the reactor shell and holed baffles 6 b placed to form openingsbetween the circumference of the reactor shell, whereby returning to thecirculation pump through circular conduits 3 b. A part of the heatmedium absorbing the reaction heat is passed through a discharge pipeprovided on the top of the circulation pump 7 with being cooled by aheat exchanger (not shown) to go into a heat medium supplying line 8 ato be re-introduced into the reactor. The adjustment of the heat mediumtemperature is accomplished by adjusting the temperature or the flowrate of the refluxing heat medium introduced from the heat mediumsupplying line 8 a to control the temperature based on the temperaturedetected by a thermometer 14.

The temperature of the heat medium is adjusted so that the difference inthe temperature between the heat medium supplying line 8 a and the heatmedium draining line 8 b is 1 to 10° C., preferably 2 to 6° C., althoughit depends on the performance of the catalyst employed.

It is preferred to provide a current plate (not shown) on the body platepart inside of the circular conduits 3 a and 3 b for the purpose ofminimizing the distribution of the heat medium flow speed toward thedirection of the circumference. As the current plate, a porous plate ora slit plate is employed, and the opening area of the porous plate orthe slit gap may be changed to achieve a rectification effect whichallows the heat medium to come in at a similar flow speed from all overthe circumference. The temperature inside the circular conduits (3 a,preferably also 3 b) can be monitored by providing a single or a pluralof thermometers 15.

While the number of the baffles provided in the reactor shell is notlimited particularly, it is preferable to provide three plates (2 platesof 6 a type and 1 plate of 6 b type) as in an ordinary case. By theexistence of these baffles, the upward flow of the heat medium isprevented, and converted into a lateral direction with respect to theaxial direction of the reactor tube, whereby allowing the heat medium tobe collected from the circumference to the center of the reactor shell,and then to be turned around toward the circumference at the opening ofthe baffle 6 a, then allowed to reach the outer cylinder of the shell.The heat medium is turned around again at the circumference of thebaffles 6 b to be collected into the center, and then goes upwardthrough the openings of the baffles 6 a to go along the upper tube plate5 a of the reactor shell toward the circumference, and then passesthrough the circular conduits 3 b to circulate to the pump.

Into a plural of the reaction tubes provided in the reactor, thethermometers 11 are inserted, and the signals are transmitted to theoutside of the reactor, and the temperature distribution in the catalystlayer in the direction of the reactor tube axis is recorded. In a pluralof the reaction tubes having the thermometers inserted thereinto, asingle thermometer can measure the temperature at 5 to 20 points in thedirection of the tube axis.

(FIG. 2, FIG. 3: Baffles)

A baffle employed in the invention may be any of a segment type chippedcircular baffle shown in FIG. 2 or a disc type baffle shown in FIG. 3,provided that a baffle 6 a has an opening near the center of the reactorshell while a baffle 6 b forms an gap between its circumference and theouter cylinder of the shell and the heat medium is turned around at eachopening to prevent any by-passing of the heat medium and to change theflow speed. In both types of the baffles, the relationship between theheat medium flow direction and the reaction tube axis is not changed.

As an ordinary baffle, a disc baffle shown in FIG. 3 is employed widely.The area of the central opening of the baffle 6 a is preferably 5 to50%, more preferably 10 to 30% of the sectional area of the reactorshell. The area of the gap between the baffle 6 b and the reactor shellbody plate 2 is preferably 5 to 50%, more preferably 10 to 30% of thesectional area of the reactor shell. A too low opening ratio of thebaffles (6 a and 6 b) leads to a too long flow path of the heat mediumwhich results in an increased pressure loss between the circularconduits (3 a and 3 b) which causes an increased power of the heatmedium circulation pump 7. A too high opening ratio of the baffles leadsto an increased number of the reaction tubes (1 c).

While the distance between the respective baffles (the distance betweenthe baffles 6 a and 6 b as well as the distance between the baffle 6 aand the tube plates 5 a, 5 b) is frequently equal, it is not necessarilyequal. It may be adjusted appropriately so that the required flow rateof the heat medium determined on the basis of the oxidation reactionheat generated in the reaction tubes is surely obtained and the pressureloss of the heat medium is low.

(FIG. 4)

FIG. 4 shows a schematic sectional view of a multi-tubular reactor inwhich the reactor shell is divided by an intermediate tube plate 9, andthe gas phase catalytic oxidation method of the invention alsoencompasses a method using a reactor of this type. In each space formedby the division, a discrete heat medium is circulated, and thetemperature is controlled discretely. While a raw material gas may beintroduced via either of 4 a or 4 b, 4 b is the raw material supplyinlet in FIG. 4 which makes the flow of the raw material process gas acountercurrent with respect to the heat medium flow since the directionof the flow of the heat medium in the reactor shell is indicated upwardby an arrow. The raw material gas introduced via the raw material supplyinlet 4 b is reacted sequentially in the reaction tubes in the reactor.

Since the temperature of the heat medium is different between the upperand lower areas (area A and area B in FIG. 4) partitioned with theintermediate tube plate 9 in the multi-tubular reactor shown in FIG. 4,the inside of the reactor is in the following three cases: 1) a case inwhich an identical catalyst is packed entirely and the reaction iseffected with different temperatures between the inlet and the outlet ofthe raw material gases of the reaction tubes, 2) a case in which thecatalyst is packed at the inlet of the raw material gases but the outletis not packed with a catalyst and allowed to be vacant or is ratherpacked with an inert substance having no reaction activity for thepurpose of cooling a reaction product rapidly, and 3) a case in whichdifferent catalysts are packed at the inlet and the outlet of the rawmaterial gases, the region between which is not packed with a catalystand allowed to be vacant or is rather packed with an inert substancehaving no reaction activity for the purpose of cooling a reactionproduct rapidly.

For example, into the multi-tubular reactor employed in the inventionshown in FIG. 4, a gas mixture of propylene, propane or isobutylene witha molecular oxygen-containing gas is introduced via the raw materialsupply inlet 4 b, and firstly converted into (meth)acrolein in the firststage for a former stage reaction (area A in the reaction tubes) andthen the (meth)acrolein is oxidized in the second stage for a latterstage reaction (area B in the reaction tubes) to produce (meth)acrylicacid. In this example, the first stage (hereinafter sometimes referredto as “former stage”) and the second stage (hereinafter sometimesreferred to as “latter stage”) of the reaction tubes are packed withdifferent catalysts, which are controlled at different temperature toeffect the reaction under an optimum condition. It is preferred that theregion where the intermediate tube plate is provided between the formerstage and the latter stage of the reaction tubes is packed with an inertsubstance which is not involved in the reaction.

(FIG. 5)

An intermediate plate is shown in FIG. 5 as being magnified. While theformer stage and the latter stage are controlled at differenttemperature, a difference in the temperature exceeding 100° C. causes anon-negligible heat transfer from a higher temperature heat medium to alower temperature heat medium which may lead to a reduced accuracy ofthe reaction temperature of the lower temperature side. In such a case,a heat insulation for preventing the heat transfer between the upper andlower regions of the intermediate tube plate is required. In FIG. 5showing the case employing the heat insulation plate, two or three heatinsulation plates 10 are provided at a position of about 10 cm below orabove the intermediate tube plate to form a stagnation zone 12 in whichthere is no flow but which is filled with the heat medium, wherebyobtaining a preferable heat insulation effect. The heat insulationplates 10 are fixed on the intermediate tube plate 9 for example byspacer rods 13.

Although the direction of the flow of the heat medium in the reactorshell is indicated upward by an arrow in FIG. 1 and FIG. 4, the reversedirection may be used in the present invention. For a decision withregard to the direction of the circulation of the heat medium, a caremust be taken to prevent any migration of a gas which may be present onthe upper edge of the reactor shell 2 and the circulation pump 7,typically an inert gas such as nitrogen, into the heat medium flow. Whenthe heat medium flows upward (FIG. 1), any migration of the gas at theupper region of the circulation pump 7 may result in a cavitation in thecirculation pump, which may lead to a worst consequence such as abreakage of the pump. When the heat medium flows downward, the gasmigration occurs at the top of the reactor shell to form a gas phasestagnation in the upper region of the shell which prevents the heatmedium from cooling the upper region of the reaction tubes around such agas stagnation.

To prevent the formation of such a gas stagnation, a gas exhausting lineshould be provided to replace the gas in the gas layer with the heatmedium. For this purpose, the heat medium pressure in the heat mediumsupplying line 8 a is increased when the heat medium flows upward(FIG. 1) and the heat medium draining line 8 b is placed as high aspossible, whereby increasing the pressure in the shell. The heat mediumdraining line is located preferably above the tube plate 5 a.

In a multi-tubular reactor which oxidizes propylene, propane orisobutylene with a molecular oxygen-containing gas, when themulti-tubular reactor shown in FIG. 1 is employed and the process gasesflow downward, i.e., the raw material gas is introduced from 4 b and theproduct is put out of 4 a, then the concentration of the intendedproduct (meth)acrolein becomes high near the product outlet 4 a of thereactor where heating by the reaction heat also raises the temperatureof the process gases. Accordingly, in such a case, it is preferable toprovide a heat exchange device following to 4 a of the reactor shown inFIG. 1 to cool the process gases sufficiently whereby preventing anyauto-oxidation of the (meth)acrolein.

Also when the multi-tubular reactor shown in FIG. 4 is employed and theprocess gases flow downward, i.e., the raw material gas is introducedfrom 4 b and the product is put out of 4 a, then the concentration ofthe intended product (meth)acrolein becomes high near the intermediatetube plate 9 at the endpoint of the reaction of the first stage (area Ain the reaction tubes) where heating by the reaction heat also raisesthe temperature of the process gases. When the catalyst is packed onlyin the first stage (area A in the reaction tubes: 5 a-6 a-6 b-6 a-9),then the reaction is not conducted in the second stage of the reactiontubes 1 b, 1 c (area B in the reaction tube: between 9 to 5 b) where theprocess gases are cooled by the heat medium flowing through the channelon the side of the shell whereby preventing any auto-oxidation of the(meth)acrolein. In such a case, it is preferable that the area B in thereaction tubes 1 b, 1 c (between 9 and 5 b) is not packed with acatalyst and is allowed to be vacant or is rather packed with a solidhaving no reaction activity. The latter is preferable for the purpose ofimproving the heat transmission profile.

Also when the first stage of the multi-tubular reactor shown in FIG. 4(area A in the reaction tubes: 5 a-6 a-6 b-6 a-9) and the second stage(area B in the reaction tube: 9-6 a′-6 b′-6 a′-5 b) are packed withdifferent catalysts to obtain (meth)acrolein from propylene, propane orisobutylene on the first stage and obtain (meth)acrylic acid on thesecond stage, the catalyst layer temperature on the first stage ishigher than that the catalyst layer temperature on the second stage.Typically, the temperature becomes higher near the reaction endpoint ofthe first stage (6 a-9) and the reaction starting point of the secondstage (9-6 a′), where it is preferred that no reaction is conducted andthe process gases are cooled by the heat medium flowing through thechannel on the side of the shell whereby preventing any auto-oxidationof the (meth)acrolein. In such a case, a zone is provided near theintermediate tube plate 9 (6 a-9-6 a′ in reaction tubes 1 b, 1 c) whichis not packed with a catalyst and is allowed to be vacant or is ratherpacked with a solid having no reaction activity. The latter ispreferable for the purpose of improving the heat transmission profile.

(Reaction Tube Diameter)

The inner diameter of a reaction tube having an effect on the gas linespeed is extremely important, since the inside of the reaction tubecontaining an oxidation catalyst in an oxidation reactor is in a gasphase, and also since the gas line speed is limited due to a resistanceby the catalyst and the heat transmission coefficient in the tube is thelowest and allows the heat transmission to be a rate determinant.

While the inner diameter of a reaction tube of a multi-tubular reactoraccording to the invention may vary depending on the reaction heatamount and the catalyst particle size in the reaction tube, it ispreferably 10 to 50 mm, more preferably 20 to 30 mm. A too small innerdiameter of the reaction tube leads to a reduced amount of the catalystto be packed which leads to an increased number of the reaction tubesrelative to the amount of the catalyst required, resulting in arequirement of a high production cost due to increased labor at the timeof the reactor production which is disadvantageous in view of anindustrial efficiency. On the other hand, a too large inner diameter ofthe reaction tube leads to a reduced surface area of the reaction tuberelative to the amount of the catalyst required, resulting in areduction in the heat transmission area for removing the reaction heat.

(Catalyst)

As a catalyst employed in a gas phase catalytic oxidation for producing(meth)acrylic acid or (meth)acrolein, there is one for a first stagereaction converting an olefin to an unsaturated aldehyde or anunsaturated acid and one for a second stage reaction converting anunsaturated aldehyde to an unsaturated acid.

In the gas phase catalytic oxidation reaction described above, anMo—Bi-based composite oxidation catalyst employed in the first stagereaction mainly for producing acrolein (reaction for converting anolefin to an unsaturated aldehyde or an unsaturated acid) may forexample be one represented by Formula (I) shown below:

Mo_(a)W_(b)Bi_(c)Fe_(d)A_(e)B_(f)C_(g)D_(h)E_(i)O_(x)  Formula (I)

In Formula (I) shown above, A denotes at least one element selected fromnickel and cobalt, B denotes at least one element selected from sodium,potassium, rubidium, cesium and thallium, C denotes at least one elementselected from alkaline earth metals, D denotes at least one elementselected from phosphorus, tellurium, antimony, tin, cerium, lead,niobium, manganese, arsenic, boron and zinc, E denotes at least oneelement selected from silicon, aluminum, titanium and zirconium, and Odenotes oxygen. a, b, c, d, e, f, g, h, i and x denotes the atomicratios of Mo, W, Bi, Fe, A, B, C, D, E and O, respectively, and when ais 12 then b is 0 to 10, c is 0 to 10 (preferably 0.1 to 10), d is 0 to10 (preferably 0.1 to 10), e is 0 to 15, f is 0 to 10 (preferably 0.001to 10), g is 0 to 10, h is 0 to 4, i is 0 to 30, x is a value determineddepending on the oxidation state of each element.

In the gas phase catalytic oxidation reaction described above, anMo—V-based composite oxidation catalyst employed in the second stagereaction for oxidizing acrolein to produce acrylic acid (reaction forconverting an unsaturated aldehyde to an unsaturated acid) may forexample be one represented by Formula (II) shown below:

MO_(a)V_(b)W_(c)CU_(d)X_(e)Y_(f)O_(g)  Formula (II)

In Formula (II) shown above, X denotes at least one element selectedfrom Mg, Ca, Sr and Ba, Y denotes at least one element selected from Ti,Zr, Ce, Cr, Mn, Fe, Co, Ni, Zn, Nb, Sn, Sb, Pb and Bi, and O denotesoxygen. a, b, C, d, e, f and g denotes the atomic ratios of Mo, V, W,Cu, X, Y and O, respectively, and when a is 12 then b is 2 to 14, c is 0to 12, d is 0 to 6, e is 0 to 3, 0 f is 0 to 3, and g is a valuedetermined depending on the oxidation state of each element.

A catalyst described above may be produced by a method described forexample in JP-A-63-54942, JP-B-6-13096, JP-B-6-38918 and the like.

A catalyst employed in the invention may be a molded catalyst obtainedby an extrusion molding or a tablet compression, or may be a supportedcatalyst formed by allowing a composite oxides consisting of catalystcomponents to be supported on an inert carrier such as silicon carbide,alumina, zirconium oxide, titanium oxide and the like.

The shape of a catalyst employed in the invention is not limitedparticularly, and may be any shape such as sphere, column, cylinder,star and ring or may be amorphous.

(Diluent)

A catalyst described above may be used as a mixture with an inertsubstance as a diluent.

While such an inert substance is not limited particularly as long as itis stable under a reaction condition and is not reactive with a rawmaterial substance or a product, it is preferably one employed as acarrier for a catalyst, such as alumina, silicon carbide, silica,zirconium oxide, titanium oxide and the like.

Its shape is not limited particularly similarly to a catalyst, and maybe any shape such as sphere, column, cylinder, star, ring, chip andnetwork or may be amorphous. The size may be determined while taking thereaction tube diameter and the pressure loss into consideration.

The amount of an inert substance as a diluent may be determinedappropriately based on the intended catalytic activity.

(Catalyst Layer, Activity Control and the Like)

The activity of a catalyst layer in a reaction tube can be changed.

A method for the adjustment for changing the activity of a catalystlayer in a reaction tube may for example be a way to adjust thecomposition of the catalysts to give catalysts having differentactivities to be used in respective catalyst layers, or a way to mix acatalyst particle with an inert substance particle to dilute thecatalyst whereby adjusting the activity of each catalyst layer.

In a typical example of the latter way, the catalyst layers consists oftwo layers, namely a low activity layer which is a catalyst layer at theinlet of the raw material gases in a reaction tube where an inertsubstance particle is contained at a higher level and the amount of theinert substance particle (ratio by mass) may for example be 0.3 to 0.7based on the catalyst, and a high activity layer which is a catalystlayer at the outlet of the reaction tube where such a ratio is as low as0 to 0.5 or a non-diluted catalyst is packed.

While the number of the catalyst layers formed in the direction of thetube axis of a multi-tubular reactor is not limited, the number of thecatalyst layers is usually 1 to 10 since a too large number of thecatalyst layers leads to a requirement of an enormous labor for packingthe catalyst. The optimum length of each catalyst layer may varydepending on the catalyst type, the number of the catalyst layers, thereaction conditions and the like, and may be determined appropriatelyfor allowing a maximum effect of the invention to be exerted.

An auxiliary matter of the invention is discussed below.

(Step for Producing Acrylic Acid or Acrylates)

A step for producing acrylic acid may for example be the steps (i) to(iii) shown below. In any step, the technique described above is taken.

(i) An oxidation step for effecting a catalytic gas phase oxidation ofpropane, propylene and/or acrolein, a collection step for bringing anacrylic acid-containing gas from the oxidation step into contact withwater to collect the acrylic acid as an aqueous solution of acrylicacid, an extraction step for extracting the acrylic acid from thisaqueous solution of the acrylic acid using a suitable extractionsolvent, and subsequent separation of the acrylic acid from the solventfollowed by a purification step are provided, and then a high boilingfluid containing a Michael adduct of the acrylic acid and polymerizationinhibitors employed in respective steps is supplied as a raw material toa decomposition reaction tower to recover valuable materials (forexample, acrylic acid) and the valuable materials are supplied to anystep of the collection step or later steps.(ii) An oxidation step for effecting a catalytic gas phase oxidation ofpropylene, propane and/or acrolein to produce acrylic acid, a collectionstep for bringing an acrylic acid-containing gas into contact with waterto collect the acrylic acid as an aqueous solution of acrylic acid, anazeotropic separation step for distilling this aqueous solution of theacrylic acid in an azeotropic separation tower in the presence of anazeotropic solvent to collect a crude acrylic acid from the towerbottom, and then an acetic acid separation step for removing aceticacid, followed by a purification step for removing high boilingimpurities are provided, and then a high boiling fluid containing aMichael adduct of the acrylic acid and polymerization inhibitorsemployed in these production steps is supplied as a raw material to adecomposition reaction tower to recover valuable materials (for example,acrylic acid) and the valuable materials are supplied to any step of thecollection step or later steps.(iii) An oxidation step for effecting a catalytic gas phase oxidation ofpropylene, propane and/or acrolein to produce acrylic acid, acollection/separation step for bringing an acrylic acid-containing gasinto contact with an organic solvent to collect the acrylic acid as anorganic solution of acrylic acid while removing water, acetic acid andthe like simultaneously, a separation step for isolating the acrylicacid from this organic solution of the acrylic acid, a step in which ahigh boiling fluid containing polymerization inhibitors employed inthese production steps, organic solvents and a Michael adduct of theacrylic acid is supplied as a raw material to a decomposition reactiontower to recover valuable materials and the valuable materials aresupplied to any step of the collection step or later, and a step forpurifying a part of the organic solvent are provided.

A step for producing an acrylate consists for example of anesterification reaction step for reacting acrylic acid with an alcoholusing an organic acid or a cationic ion exchange resin as a catalyst anda purification step for conducting extraction, evaporation anddistillation each as a unit operation for condensing the crude acrylatesolution obtained in the reaction. Each unit operation may be selectedappropriately based on the ratio of the acrylic acid and the alcohol asraw materials in the esterification reaction, the type of the catalystemployed for the esterification reaction, or the physical properties ofthe raw materials, reaction by-products and acrylates. Throughrespective unit operations, a product is obtained in an acrylatepurification tower. The fluid on the bottom of the purification towermay be supplied to a decomposition reaction tower as a high boilingfluid containing a Michael adduct whose main components are acrylates,β-acryloxypropionates, β-alkoxypropionates, β-hydroxypropionatestogether with polymerization inhibitors employed in the productionsteps, or may be returned to the process whereby recovering the valuablematerials.

In the production of acrylic acid or acrylates which are readilypolymerizable compounds, a polymerization inhibitor is employed tosuppress the formation of the polymers during the production.

Typically, such a polymerization inhibitor may for example be copperacrylate, copper dithiocarbamate, phenolic compounds, phenothiazinecompounds and the like. The copper dithiocarbamate may for example be acopper dialkyldithiocarbamate such as copper dimethyldithiocarbamate,copper diethyldithiocarbamate, copper dipropyldithiocarbamate, copperdibutyldithiocarbamate and the like, a coppercycloalkylenedithiocarbamate such as copper ethylenedithiocarbamate,copper tetramethylene dithiocarbamate, copper pentamethylenedithiocarbamate, copper hexamethylenedithiocarbamate and the like, and acopper cyclooxydialkylenedithiocarbamate such as copperoxydiethylenedithiocarbamate and the like. The phenolic compound may forexample be hydroquinone, methoquinone, pyrogallol, cathecol, resorcine,phenol, cresol and the like. The phenothiazine compound may for examplebe phenothiazine, bis(α-methylbenzyl)phenothiazine,3,7-dioctylphenothiazine, bis(α-dimethylbenzyl)phenothiazine and thelike.

While substances other than those listed above may be involved in someprocesses, their types clearly have no effects on the invention.

Acrylic acid or acrylates thus obtained can be used in variousapplications. Typically, it may be used in a highly absorptive resin,coagulant, pressure-sensitive adhesive, paint, adhesive, fiber modifierand the like.

EXAMPLES

The invention is further described in the following Example andComparative Example which are not intended to restrict the invention.

Example 1 (Catalyst)

94 Parts by mass of antimony paramolybdate was dissolved in 400 parts bymass of pure water with heating. On the other hand, 7.2 parts by mass offerric nitrate, 25 parts by mass of cobalt nitrate and 38 parts by massof nickel nitrate were dissolved in 60 parts by mass of pure water withheating. These solutions were mixed with stirring thoroughly to obtain aslurry solution.

Then, 0.85 parts by mass of borax and 0.36 parts by mass of potassiumnitrate were dissolved in 40 parts by mass of pure water with heatingand then added to the slurry described above. Then 64 parts by mass ofparticulate silica was added and stirred. Then 58 parts by mass ofbismuth subcarbonate which had previously be made composite with 0.8% bymass of Mg was added and mixed with stirring, and this slurry was diedwith heating, and then heat-treated for 1 hour at 300° C. in airatmosphere, and the resultant particulate solid was subjected to tabletcompression using a molding machine into tablets each being 5 mm indiameter and 4 mm in height, and then sintered for 4 hours at 500° C. toobtain a former stage catalyst.

The former stage catalyst thus obtained was an Mo—Bi-based compositeoxide having the composition ratio of a catalyst powder whose formulawas Mo₁₂Bi₅Ni₃Co₂Fe_(0.4)Na_(0.2)Mg_(0.4)B_(0.2)K_(0.1)Si₂₄O_(x) (theoxygen composition ratio x is a value determined depending on theoxidation state of each metal.

(Production of Acrylic Acid and Acrolein from Propylene)

In this Example, a multi-tubular reactor similar to that shown in FIG. 1was employed.

Typically, a multi-tubular reactor of a reaction shell (inner diameter:4,500 mm) having 10,000 stainless steel-made reaction tube each being3.5 m in length and 27 mm in inner diameter was employed. No reactiontube was provided in the round opening region in the center of a holeddisc baffle 6 a having an opening near the center of the reactor shell.The baffles consisted of holed disc baffles 6 a each having an openingnear the center of the reactor shell and a holed disc baffle 6 b whichformed a gap between the circumference of the reactor, which werelocated at equal intervals in the order of 6 a-6 b-6 a, with the openingratio of each baffle being 18%.

A catalyst to be packed in each reaction tube was obtained by mixing theformer stage catalyst described above with silica-made balls each havingno catalytic activity and being 5 mm in diameter to adjust the catalyticactivity, and packed in such a manner that the ratio of the catalyticactivities became 0.5, 0.7 and 1 from the inlet of the reaction tube,whereby forming three catalyst layers.

(Start-Up Method)

A heat medium (NITER) which was an inorganic mixed salt was passedthrough the side of the reactor shell to keep the temperature at 330° C.Prior to the supply of propylene, 1845 Nm³/hr of oxygen, 8241 Nm³/hr ofnitrogen and 1107 Nm³/hr of water vapor were supplied to the reactor andthen the catalytic layer temperature was ensured to be almost similar tothat of the NITER, and thereafter the supply of propylene was started.

The propylene supply was reached 340 Nm³/hr 2 hours after the start, andthen the supply amount was increased by 50 Nm³/hr per hour to reach 775Nm³/hr (corresponding to about 70% of the maximum supply amount) about11 hours after the start. The NITER temperature was kept at 330° C. for12 hours.

Then the propylene supply amount was increased over about 70 minutesuntil 830 Nm³/hr (corresponding to 75% of the maximum supply amount).The NITER temperature was kept at 331° C. for 24 hours.

Then the propylene supply amount was increased over about 200 minutesuntil 996 Nm³/hr (corresponding to 90% of the maximum supply amount),and the NITER temperature was kept at 333° C. for 4 hours, followed byan elevation to 1107 Nm³/hr (corresponding to 100% of the maximum supplyamount) over about 130 minutes, and then the NITER temperature was setat 335° C. for switching into a stationary operation.

At this time, the raw material gas composition consisted of 9% by moleof propylene, 15% by mole of oxygen, 9% by mole of water vapor, 67% bymole of nitrogen, with the pressure being 75 kPa (gauge pressure) andthe gas supply amount being 12300 Nm³/hr.

(Stationary Operation)

When operating for a prolonged period, the NITER temperature wasadjusted so that the % propylene conversion became 97%. The NITERtemperature after 1 year was 337° C. During this period, the total yieldof acrolein and acrylic acid was 92%.

One year after this stationary operation, the reactor was opened and 84reaction tubes in total were removed from the regions near the center ofthe reactor shell, near the periphery and intermediate zone inside theshell at an almost same radial angle, and examined macroscopically, andno abnormality was observed in each removed catalyst.

Comparative Example 1

The procedure similar to that in Example 1 including the stationaryoperation was conducted except for setting the propylene supply amountat 1107 Nm³/hr (corresponding to 100% of the maximum supply amount)within 15 hours after the start. The NITER temperature was targeted to atemperature corresponding to the maximum supply amount ratio in Example1.

Once the propylene supply amount exceeded 900 Nm³/hr, the temperature ofthe catalyst layers could not be kept at a constant value. Since theNITER temperature became impossible to be kept at a prescribedtemperature, it was set at a temperature lower by 1 to 2° C. After thepropylene supply amount became 1107 Nm³/hr and the temperature of thecatalyst layers became stable, the NITER temperature was set at 335° C.to terminate the start-up operation.

The % propylene conversion in the stationary state was not higher than96.5%, and the total yield of acrolein and acrylic acid was 89%.

Since the % propylene conversion was low during the stationaryoperation, the operation was discontinued after 1 month, and the reactorwas opened and the reaction tubes were removed and examined in themanner similar to that in Example 1 described above. As a result of theexamination, a part of the catalyst removed from the reaction tubes nearthe center of the reactor and near the circumference of the reactorexhibited a deactivation which was observed macroscopically (thecondition similar to the color and the shape (shrinkage) shownempirically by a deactivated catalyst).

While the invention has been described in detail with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and the scope thereof.

This application is based on the Japanese patent application filed onMay 26, 2004 (Patent Application No. 2004-155840), the entire contentsthereof being hereby incorporated by reference.

INDUSTRIAL APPLICABILITY

According to a production method of the invention, (meth)acrylic acid or(meth)acrolein can be produced at a higher yield and more stably evenwhen supplying the raw material in an amount close to the maximum supplyamount acceptable by a reactor. The resultant acrylic acid or acrylatescan be used in a highly absorptive resin, coagulant, pressure-sensitiveadhesive, paint, adhesive, fiber modifier and the like.

1. A method for producing (meth)acrylic acid or (meth)acrolein byconducting a gas phase catalytic oxidation reaction with anoxygen-containing gas using as a raw material at least one substance tobe oxidized selected from propylene, propane, isobutylene and(meth)acrolein using a multi-tubular reactor, wherein, at the time of astart-up of the reaction, for a period of at least 20 hours or moreafter the supply amount of the raw material to the reactor per unit timereached 30% or more of the acceptable maximum supply amount of the rawmaterial per unit time, the supply amount of the raw material per unittime is kept at 30% or more and less than 80% of the acceptable maximumsupply amount.